"Sweeney came in, did a really good job for the first two months here, then got hurt and was playing hurt for a while and unfortunately his season ended," general manager Ben Cherington said.
The 27-year-old played in 63 games for the Red Sox, and had three stints on the disabled list. He hit .260 with 18 doubles, two triples, no homers and 16 RBIs.
Daniel Nava and Ryan Kalish are alternatives to help supplement the outfield. The Red Sox also signed left fielder Jonny Gomes to a two-year, $10 million deal on Saturday, and they are trying to re-sign Cody Ross.
The Sox also non-tendered veteran relievers Scott Atchison and Rich Hill, but there's a chance both could return on Minor League deals that include invites to Spring Training.
"All three guys have done really good things for us while they were here, and certainly Atchison has been a really good performer and he's a great teammate," Cherington said. "He's a guy we'd like to continue to talk to and see if there's a way to bring him back. Same with Rich Hill. Rich has missed a lot of time the last couple years with injuries, but he's a proven left-handed pitcher and certainly capable of helping the Major League bullpen."
In fact, Hill was non-tendered last winter and wound up returning.
Atchison, a righty, has been an underrated performer for the Red Sox in recent years. In 2012 he went 2-1 with a 1.58 ERA, notching 36 strikeouts. He missed 55 games because of an injured UCL in his right elbow but wound up not needing surgery.
Injuries have also been a problem for left-hander Hill, a native of Milton, Mass. Hill bounced back from the Tommy John surgery he had in June 2011 but was able to pitch in only 25 games because of a strained flexor in his elbow. Hill has pitched well for the Red Sox when healthy.
Sweeney, Hill and Atchison are now free agents.
"It's just sort of the nature of the arbitration process," Cherington said. "Sometimes all the stars don't align, when you're sort of factoring in playing time, what they've done, what they might make for arbitration, it didn't add up right now. We needed to make the decision, but we'll keep the door open with all three guys."
The Sox did tender contracts to the other 30 unsigned players, a list that includes Jacoby Ellsbury, Alfredo Aceves, Jarrod Saltalamacchia and Daniel Bard.